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and task did not engender a highly focused state of attention and that some information about the to-be-ignored stimuli “leaked through” the attentional filter. Indeed, other investigators have provided examples of experiments in which attention was highly focused and no interference from the to-be-ignored stimuli was present, from which they concluded that the to-be-ignored stimuli were not perceived (14, 15). However, this conclusion is also problematic because it is possible that the to-be-ignored stimulus was identified but then blocked from influencing behavior at a postperceptual stage (as argued in refs. 16 and 17). Thus, neither the presence nor the absence of interference provides clear evidence about the locus of selection.

Neurophysiological Evidence for Early Selection

The main difficulty in determining the locus of selection from studies of behavior is that behavior reflects the output of processing and does not directly reveal the individual steps that led to that output. The techniques of cognitive neuroscience, however, naturally tend to subdivide processing into different stages on the basis of neuroanatomy and/or timing. For example, if attention can be shown to influence the initial neural activity in sensory processing regions, then this would provide clear evidence for early selection. In contrast, if attention influences only late neural activity in high-level processing regions, then this would provide clear evidence for late selection (assuming that the stimuli and task engendered a highly focused attentional state). Note, however, that it is important to assess both the timing and the neuroanatomical site of the effects of attention. For example, finding that attention influences neural activity in primary visual cortex does not necessarily indicate that attention operates before stimulus identification is complete, because it is possible that neurons in this area also participate in postperceptual processes such as working memory. Similarly, finding that attention begins to modulate neural activity beginning 150 ms after stimulus onset does not prove that attention modulates perceptual processing, because it is possible that postperceptual processes have begun by this time. Extreme cases, however, may provide fairly compelling evidence. In particular, any effects of visual attention observed before 100 ms poststimulus or observed in the retina are very likely to reflect modulations of perceptual processing. In addition, attention effects that occur at a relatively early time and at a relatively early neuroanatomical locus are likely to reflect early selection.

Cognitive neuroscience techniques have been applied to the study of visual attention for more than 20 years, and these studies have generally indicated that focusing attention onto a location in space leads to a modulation of perceptual processing, although not until a significant amount of early sensory analysis has taken place. This conclusion is based on event-related potential (ERP) and positron emission tomography (PET) studies in humans and single-unit recordings in monkeys. Many of these studies have used variations on the paradigm shown in Fig. 1A. In this paradigm, the subjects are instructed to attend to one location during some trial blocks and to a different location during others, and they are required to detect occasional target stimuli at the attended location that are interspersed among nontarget stimuli presented at both attended and unattended locations. As shown in Fig. 1B, the initial “C1” wave of the ERP waveform is not influenced by whether the evoking stimulus is presented at the attended location or at the ignored location, but the subsequent “P1” and “N1” waves are larger for the attended-location stimuli. Several studies have indicated that the C1 wave is generated in area V1 (1820), so the absence of an attentional modulation of this component suggests that attention operates after this very early stage of processing. A recent PET study has indicated that the P1 wave arises in extrastriate areas of visual

FIG. 1. (A) Common experimental design for neurophysiological studies of attention. The outline squares are continuously present and mark the two locations at which the solid square can be flashed. (B) Example occipital ERPs recorded in a paradigm of this nature (data from ref. 42). Note that the C1 wave (generated in area V1) shows no attention effect, whereas the P1 and the N1 waves (generated in extrastriate cortex) are larger for the attended stimuli. (C) Single-unit responses from area V4 in a similar paradigm (data from ref. 23). Note that the response is larger for attended compared with ignored stimuli. (D) Single-unit responses from area V1 (data from ref. 23) showing no effect of attention.

cortex (21), and the combination of this anatomical information with the fact that the P1 attentional modulation begins before 100 ms provides excellent evidence that visual-spatial attention influences perceptual processing. It is also important to note that these effects are identical for both target and nontarget stimuli (22), which provides further evidence that attention operates before perceptual processing is complete.

More precise evidence has been obtained from single-unit recordings in area V1 (primary visual cortex) and in area V4 (an intermediate stage in the object recognition pathway) (23). As shown in Fig. 1C and D, responses in area V4 were found to be larger for attended-location stimuli than for ignored-location stimuli, but no effects of attention were observed in area V1. Importantly, both the initial stimulus-evoked activity

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